提议本地数据声明/实例

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问题描述

我很好奇，并且一直无法在Haskell中找到这样的提议。考虑是否已经编写 sort ，但不是 sortBy 。

  sortBy :: forall a。 （a  - > a  - >订购） - > [a]  - > [a] 
 sortBy f = map getX。排序。 map X 
其中
 newtype X = X {getX :: a} 
实例Ord X其中
比较（X a）（X b）= fab

有没有人看过这个建议？

解决方案 
I'm curious, and have been unable to find a proposal for something like this in Haskell.  Consider if sort had been written but not sortBy.  
sortBy :: forall a. (a -> a -> Ordering) -> [a] -> [a]
sortBy f = map getX . sort . map X
    where
    newtype X = X { getX :: a }
    instance Ord X where
        compare (X a) (X b) = f a b
Has anyone seen this proposal?
 解决方案 
Apparently, local instances have been discussed briefly at Haskell Prime mailing list: http://web.archiveorange.com/archive/v/eKcS7T2qBpy7czBE2Jei, and more fully in the 6 chapter of Oleg’s paper "Functional Pearl: Implicit Conﬁgurations". I have heard practically nothing about local data declarations, though.
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